【作者】 陈梅英；

【导师】 陈锦权；

【作者基本信息】 福建农林大学 ， 农产品加工及贮藏工程， 2009， 博士

【摘要】 冷冻浓缩产生的冰晶夹带率比较高,且难以分离,从而限制了它的推广与使用。长期以来,为解决冰晶夹带问题,国内外学者大多把目光停留在单位冰晶体积的层面上,主要凭经验或以实测资料为依据来控制冰晶的生长过程。本文从冷冻浓缩冰晶夹带率影响因素入手,先从宏观上分析冰晶生长的机制,再借鉴国内外金属微观结构研究领域非常热门的相场模型,微观上探索冰晶组织的形成规律和控制方法,较为系统地展开对冷冻浓缩的研究。具体的研究工作和主要结论如下:1.以荔枝汁为原料,讨论分析冰晶纯度影响因素。主要探讨冷冻温度、冷冻时间、冰晶形态、溶液粘度、冰晶分离工艺等方面对冰晶夹带率的影响。选择初始冷冻温度、把握冷冻时间、控制冰晶形态和采取合适的冰晶分离工艺,从而一定程度上降低冰晶的夹带率。2.通过对冷冻时间与浓缩液、冰晶中可溶性固形物含量的关系以及冷冻时间与浓缩液和冰晶质量关系进行研究,并根据Fick扩散方程式建立冰晶增长动力学模型,模型显示冰晶增长速率与冰晶质量成正比,同时也受到最大冰晶量的抑制。通过该模型,从宏观方面分析冰晶质量增长和热量传递之间的平衡,为建立冰晶增长的质量、热量传递之间的平衡提供了一定的理论依据和试验参考,从而把握较为合理的冰晶生长时间。3.冰晶是典型的凝固组织,因其非平衡组织结构涉及到热量、质量和动量传输以及界面动力学和毛细作用效应相耦合的自由边界问题。相场模型将组织转变过程界面演化的尖锐界面问题转变为弥散界面模型,故引入相场模型,用于模拟冷冻浓缩冰晶生长过程中微观组织的演化过程。探讨了相场模型建立的基本原理、其数值求解的离散处理、稳定性条件、初始条件和边界条件。4.利用相场模型对过冷纯水的冰晶生长过程进行数值模拟,模拟不同过冷时间、不同过冷度下的冰晶形貌的演化过程,并解释了Ostwald效应的竞争机制。同时与实际拍摄的冰晶进行比较,进一步证实相场法从微观上模拟冰晶生长的可行性。5.利用相场模型对液态食品冷冻浓缩过程的冰晶生长进行数值模拟。把液态简化为糖水二元物质,在近似等温的条件下耦合相场和溶质场进行模拟,探讨了冷冻时间、过冷度对冰晶形貌及溶质场的影响。验证控制冷冻时间、冷冻温度对减少冰晶夹带率的作用。更多还原

【Abstract】 The entrainment rate of ice crystals was relatively high in the process of freeze concentration and difficult separation, and the seperation of ice crystles and soluble solid content is difficult, thereby it was limited to promote and use. Since a long time, in order to address the issue of entrainment of ice crystals, the unit size of ice crystals was concerned by domestic and foreign experts and scholars who con- trolled the growth of ice crystals based on the measured datas or main experience. In this paper, the growth mechanism of ice crystals was analyzed at the micro-macro level in the process of freeze concentration. Through domestic and international field of metal micro-structure of the phase-field model, the law of formation of ice crystals and methods of controlling were explored. Specific research work and the main con- clusions were as follows:1、The influencing factors of ice crystal purity were discussed and analyzed with litchi juices as raw materials. It was focused on the effects of the freeze temperature, freezing time, ice crystals form, the solution viscosity and separation process on the entrainment rate of ice crystals. To some extent, the entrainment rate of ice crystals could be reduced by controlling freezing temperature, freezing time, ice crystal form and separation process.2、The relationship among freezing time, concentrated liquid and soluble solid content was studied, and the relationship among freezing time, concentrated liquid and ice crystal quality was also studied. According to Fick diffusion equation, The kinetic model of ice crystals growth was developed, ,max,which showed that growth rate of ice was proportional to the mass of ice and was inhibited by the maximal mass of ice. The balance between the quality of ice crystal growth and heat transfer was analyzed at the macro level through the establishment of dynamic model of freeze concentration. Thus, a theoretical basis was provided for the establishment of the balance between the quality of ice crystal growth and heat transfer,and a more reasonable time for ice crystal growth was obtained.3、Ice crystal was a typical kind of solidfication structure. The phase-field model was introduced because of its non-equilibrium from its typical structure, involving energy, quality and momentum transfer, as well as the interface kinetics and the problem caused by capillarity effects coupled with the free boundary. Taken advantages of phase-field model which could change the sharp interface from the course of interface evolution into the diffuse interface, it was used to simulate the process of evolution of microstructure in the process of ice crystal growing of freeze concentration. The basic principles of phase-field,the discrete numerical solution, stability conditions, initial conditions and boundary conditions were discussed.4、The growth process of ice crystals of supercooling water was numerically simulated by phase-field model, which including the course of evolution of ice crystal morphology at different cooling time, different degrees of supercooling. And the competitive mechanism of Ostwald effect was explained. At the same time, compared with the actual filming ice crystals, it was confirmed further that to simulate ice crystal growth by phase-field model at the micro level was feasible.5、The growth of ice crystals was numerically simulated in the process of freeze concentration of liquid food. Liquid food were simplified as the dual material. Then it was simulated by coupling phase field with solute field under the isothermal condition. the effects of ice crystal morphology and solute field on freezing time and degrees of supercooling were studied. As a result, the control of freezing time and freezing tem- perature played an important role in reducing the entrainment of ice crystals.更多还原